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[SOLVED] Crystal needed for MCU controlling Tx module

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Junior Member level 3
Aug 26, 2011
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Hi Everyone,

I was wondering if anyone had any experiences with operating transmitters modules with an MCU. I have a circuit that works great with a PIC16F883 and an OOK 433 MHz transmitter module. I am using the internal oscialltor in the PIC. I was wondering if it will make any difference if I use a crystal or the internal oscillator.

I think the main question is, when using the int osc, will my data pattern be keyed to the transmitter with reliable timing or will it drift to an unuseable point over time? I am unsure wether to place a crystal on my PCB and wether or not it is even required??

Any help would be very much appreciated.


The transmitter module is analog and it doesn't have a synchronous interface. So, you can transmit at any rate to your heart's content, as far as the transmitter is concerned. (up to the maximum bitrate defined for your transmitter module.
(The Tx module may have a crystal, but that doesn't mean it requires clocked data; the crystal is being used to set the transmit frequency, which is a different thing).

i think the internal rc osc drifts not more than maybe around +/- 1 % or so. Maybe a bit more.

So your comms, which i presume must be async, would drift by that amount, which is tolerable.

However it also depends on the baudrate you have chosen, since the dividers may already be off by some margin depending on this baudrate vs osc frequency.

the good news is that if your async baudrate is not too high (what is it btw ?), then you will probably be ok.

Do check the datasheets for these issues though.
Thanks for the replies guys.

The baud rate is 4000 bits per second. It is pretty much like a wireless doorbell signal, all I have to transmit is a control signal ( it is for my central heating ). I don't want to hard wire it all and then it stops working. From the sounds of I think I should be ok?

yeah i suppose so... but 4000baud ? strange number......

Hi Everyone,

Thanks for your help with this one. I have left the XT out and used the internal oscillator with no problems.

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